1. Field of the Invention
The present invention is related to a method of performing attach procedures, and more particularly, to a method of performing attach procedures by camping on one or more public land mobile networks using one or more radio access technologies.
2. Description of the Prior Art
3GPP (the 3rd Generation Partnership Project) wireless communication systems utilize WCDMA (Wideband Code Division Multiple Access) techniques and is capable of providing better performance in frequency efficiency, radio coverage, communication quality and transmission speed for multi-media applications. Also, 3GPP wireless communication systems can satisfy different QoS (Quality of Service) requirements to provide a flexible bi-directional transmission service with better communication quality and low call drop rate.
Common radio access technologies include GSM (Global System for Mobile communications), GERAN(GSM/EDEG Radio Access Network), CDMA(Code Division Multiple Access), UMTS(Universal Mobile Telecommunications System), LTE (Long Term Evolution), and E-UTRAN (Evolved Universal Terrestrial Radio Access Network) systems.
A UMTS 3GPP wireless communication system includes a user equipment (UE), a UMTS terrestrial radio access network (UTRAN) and a core network (CN). The user equipment may include a mobile equipment (ME) and a UMTS subscriber identity module (USIM), or includes a mobile equipment and a subscriber identity module (SIM). The user equipment may be connected to a Node-B of the wireless access network for communicating with a radio network controller (RNC). Via its CS (Circuit Switched) domain or PS (Packet Switched) domain depending on service type, the core network may be connected to PSTN (Public Switched Telephone Network), ISDN (Integrated Services Digital Network) or Internet for providing various voice or data services. In CS domain which evolves from GSM network, a location area (LA) including multiple cells is configured to process voice signals for providing CS services. In PS domain which evolves from GPRS (General Packet Radio Service) network, a routing area (RA) including multiple cells is configured to process data signals for providing PS services.
In the prior art, when roaming to a PLMN (Public Land Mobile Network), a user equipment may request PS services by performing an attach procedure using a first RAT (Radio Access Technology) which provides the best signal quality. However, if the PLMN can only provide PS services via a second RAT, it then sends an attach reject message to the user equipment, which thus determines that PS services cannot be supported by its USIM or SIM. In the prior art, even if the PLMN can provide PS services via the second RAT which provides worse signal quality, the attach reject message associated with the first RAT prevents the user equipment from receiving PS services.